1. Field of the Invention
This invention relates to a flexible printed circuit to be used for wiring a hard disk drive (HDD) or the like. This invention particularly relates to a flexible printed circuit provided at lower-cost and having a longer fatigue life
2. Description of Related Art
Conventionally, flexible printed circuits (FPC) have recently been used extensively as wiring boards for printer heads, movable components like suspensions for hard disk drives (HDD), etc., because an FPC is thick, flexible, and has good fatigue life.
FIG. 1 is a cross section of this kind of FPC. Reference numeral 10 is a flexible printed circuit. This FPC is made in such a way that copper foil is put on a base film 1 by using a bonding agent 2; then, etching is performed on this copper foil, a circuit 3 is formed by leaving a part which is from a wiring and removing the rest of the part, and furthermore, a cover layer 5 is attached on this circuit 3 via bonding agent 4.
For such an FPC 10, long fatigue life is required so that disconnection due to the breaking of the circuit 3 does not occur due to the movement of the movable component, even if the FPC is bent more than tens of millions of times.
For this reason, a plastic film made of polyester resin such as polyimide resin and polyethyleneterephthalate (PET) which are excellent in flexibility are used for the base film 1 and the cover layer 5.
Also, the copper foil with very high purity such as rolled annealed foil (RA foil) which is superior in fatigue life, and low recrystallization temperature foil (HTE foil) is favorably used as the copper foil to be used for the circuit 3
The rolled annealed foil is made by repeatedly drawing a copper ingot with high purity in a roll strip mill and annealing it; thus, the rolled annealed foil is an intensively machined product, and has excellent fatigue life.
Also, the recrystallization temperature of the copper foil made of low recrystallization temperature foil is low; thus, the low recrystallization temperature foil has features that the recrystallization easily occurs even by a slight deformation, and relieves the deformation; thus, the low recrystallization temperature foil has high flexibility.
For FPC 10 in which particularly superior fatigue life is required, rolled annealed foil is used in most cases.
However, obtaining low cost FPC 10 is difficult because rolled annealed foil is expensive.
Also, in the case in which the bending is repeated and the bending diameter is small, even the rolled annealed foil will lose flexibility by fatigue, and finally the disconnection of the circuit 3 occurs because of cracking. This fatigue in the copper foil is mainly caused by lattice defects such as irreversible transmission which occurs when metallic organization receives plastic deformation. Such a defect is thought to form a crack with accumulating fatigue, and therefore the crack expands causing the disconnection.
This invention was made in view of the above condition, and an object of this invention is to provide a flexible print circuit which is made at lower cost and has superior fatigue life.